Dynamics of Deep Time - Species
The Species Concept - Evolution, Extinctions, and Transformations
Origin and Definition of Species
Current Thought on the Time of Life's Emergence
Theories on the emergence of life span a wide spectrum, from abiogenesis hypotheses suggesting life arose from non-living matter, to panspermia theories proposing life arrived on Earth from extraterrestrial sources.
Understanding the Species Concept
Defining Species
The definition of a species remains a contentious topic in biology. Traditionally, a species is considered a group of organisms capable of interbreeding and producing fertile offspring. However, this definition becomes complicated when applied to organisms like viruses or asexual organisms.
Viruses and Asexual Organisms
Viruses challenge the traditional species concept as they lack cellular structure and reproduce only within host cells. Asexual organisms, which reproduce without the need for genetic recombination, further blur the lines of traditional species boundaries.
Geographic and Temporal Clines
Geographic Variation and Clines
Geographic variation refers to the differences in traits observed among populations of a species distributed across different geographical areas. These variations can create clines, gradual changes in traits along a geographical gradient.
Temporal Variation and Clines
Temporal variation involves changes in traits over time. This can lead to temporal clines, where traits of a population shift gradually over generations due to environmental or other selective pressures.
Evolutionary Dynamics and Temporal Variations
Temporal Variations in Species Evolution
Species are not static entities; they evolve over time. Temporal variations manifest as species transition into new forms through processes like natural selection, genetic drift, and gene flow. Punctuated equilibrium theory suggests that evolutionary change occurs in rapid bursts separated by long periods of stasis.
Extinct Species Estimates
Total Number of Extinct Animal Species
Estimating the total number of extinct animal species is challenging due to incomplete fossil records and the difficulty of distinguishing between species. Lower and upper bound estimates vary widely, influenced by factors such as extinction rates and fossilization probabilities.
Duration of Species Existence
The duration of a species is not fixed, as species often evolve into successor species over time. Lower and upper bounds for the duration of a given species reflect this evolutionary continuum, where boundaries between species are often blurred.
Extinction without Progeny
Species can vanish without leaving progeny, especially in cases of sudden environmental changes or catastrophic events. Such extinctions contribute to turnover rates in biodiversity.
Extant Species Estimates
Total Number of Extant Species
Estimating the total number of extant species is a daunting task due to the vast diversity of life on Earth and the discovery of new species. Lower and upper bounds for the number of extant species vary across different taxonomic groups, including animals, plants, bacteria, and virus varieties.